1. Field of the Invention
The present invention relates to polyurethane powder enamels derived from isophorone diisocyanates that are free of cleavage products, which have very good storage stability, and which cure above 120.degree. C. to form impact resistant, glossy, weather resistant enamel films, as well as to the enamel coatings produced from the powders so prepared.
2. Description of the Prior Art
From German OS No. 25 02 934 it is known to produce PUR-powder enamels, in which polyisocyanates that contain latent isocyanate groups present at the same time both as uretidinedione groups and as terminal isocyanate groups blocked with blocking agents, are cured with polyols. Depending on the particular blocking agent used, more or less extensive environmental pollution occurs during the curing process. Furthermore, in this case, the functional groups of the cross-linking component play no part in the production of the enamel.
Additionally, in German OS No. 24 20 475 polyurethane powder enamels are described that are produced by the reaction of polyisocyanates having uretidinedione groups with polyols. However, the examples are limited to the uretidinedione dimer of toluene diisocyanate and the uretidinedione dimer of hexamethylene diisocyanate. Also it is necessary in this case to compensate for the higher functional portion of the cross-linking agent with monofunctional reagents, i.e., the uretidinedione/isocyanurate mixtures must first be linearized.
With the dimerizaton catalysts belonging to the prior art (tertiary phosphines) it has hitherto not been possible to produce an isocyanurate-free uretidinedione dimer of isophorone diisocyanate (IPDI). A uretidinedione dimer of isophorone diisocyanate produced in accordance with German OS No. 16 70 720 or German OS No. 19 34 763 contains an additional ca. 20-40% by weight of the trimeric IPDI (isocyanurate trimer of isophorone diisocyanate) in the mixture, depending on the reaction conditions.
Thus, it is not surprising that until now no polyurethane powder enamels free of cleavage products were known that were prepared from polyols and linear polyisocyanate polyaddition compounds which are irreversibly blocked, partially or completely, and which contain uretidinedione groups, because the prerequisite for producing the above-mentioned polyaddition compounds containing uretidinedione groups is that the uretidinedione-containing diisocyanate added to extend the chain by reacting with the diols must have an NCO functionality of 2. When the NCO functionality of the uretidinedione-containing diisocyanate is &gt;2 at least some gelling must be expected when it is reacted with diols.
Therefore the known uretidinedione dimer of isophorone diisocyanate, which contains isocyanurate, cannot be considered when further reaction with diols is intended to prepare useful starting materials for polyurethane chemistry (binding components for PUR powders free of blocking agents). Thus an isocyanate-free uretidinedione dimer of isophorone diisocyanate is still needed.